Purification of ferricyanide



Patented Jan. 9, 1945 Louis` L. `Lento, Jr.,` and Alfred G. `Houpt,Springf V dale,"Conn., assignors to, American Cyanamid H Company, NewMaine York,` N.` Y., a corporation of` Application April `2, 1941,SerialNo. 386,424

`(CLES-84:)

4 even` ammonium ferricyanide solutions and the 17 Claims.

`This invention relates to a process for the purication of ferricyanidesolutions. More particularly it embraces `a novel method for clarifyingsolutions of sodium ferricyanide, especially when contaminated withdecomposition products and the products of undesirable side reactions.

Heretofore it has been difficult, if not impossible, to` remove suchundesirable products as the blue-grec iron complexes frequently found inthe ferricyanide compounds commonly used in the photographic arts and inblue printing processes. These impurities are usuallypresentin the formof very iinely divided solids Which resemble colloidal complexes intheirdiflcultyof removal.` In addition, these ferro-ferricornplexesdiscolor the ierricyanide solution causingiitto haveva muddy blue-greenhue instead of the characteristically dark blood-red or ruby-red colorof the` pure solutions and crystals of ferricyanide. Furthermore, thepresence of these undesirable bluegreen complexes is particularlydisadvantageous since they are partially precipitated `with theferricyanide salts during the 'recovery of the latter l `and `especiallywith the double salts `heretofore found particularly desirable for use`in photolike,` or mixtures thereof, by treating them with causticsodain excess-or a substance capable of` yielding a strongly alkalinesolutionsuch as so-` dium carbonate or ammonium hydroxide, and

thelike in order to decompose the blue-green iron i `complexespresentand -convert them into ferrie V `hydroxide which is then `readilyremoved by fil--1 tration. i

The flow sheet diagrammatically illustrates the process.

l In general, thecommerci'al production of so dium ferricyanide iscarried` out by oxidizing sodium ferrocyanide by` meansof chlorine.During this chlorination step, chlorides such as sodium chloride `andcomplex ferro-ferri color bodies graphic and blue printinglprocesses. Asa result` of this simultaneous precipitation of desirable andundesirable compounds, the crystals of the extracted ferricyanideproduct are obtained with a coating of4 the blue-green ferro-terridecompositionproducts. This mixed product yields i excessively turbidaqueous solutions which `vary from blue-greento olive green insteadofthe more desirable golden yellow to'orange-red color deemed moredesirable `in commercial usage.

Moreover, blue prints preparedlfrom such impure` or contaminatedferricyanides have been found l to undergo undesirable color changesWhile being developed and the resultant prints are'` frequently mottledand speckled and in addition "iade more rapidly than those prepared froma purer product.

" `The principal object of this invention .iis to avoid thesedisadvantages ofthe prior commercial practices. Furthermore, it is` alsoan object hereof to prepare an alkali or a double alkali l metalferricyanide by a simple, direct and eflicient process. Stillanotherobject is to prepare pure sodium ferricyanide as an intermediate in arelatively` inexpensive :method of preparing a `,double alkalimetalferricyanidecontaining potassium. 1 Theinventionin itsbroadest aspectcontemplates thepurication of crude alkaline forming metal` and4ammonium ferricyanides, suchfor example as crude sodium, potassium,"calcium and are formed. The complex ferro-ferri color bodies and otherinsolubles resulting from the chlorination step are undesirableimpurities. moval is effected by adding enoughV of an alkaline substance:to `raise the pH of the solution to 9.3 or higher. This decomposes theferro-ferri complexes andconverts them into ferrie hydroxide or similareasily removable compounds. A lltration step is then utilized to removethe iron hydroxide and other insolubles. i i The color bodes may beprevented from forming in the original chlorination step by maintainingthe pH of the solution during the chlorination from 8 to9. However,after the color bodies have l been formed, it isnecessaryto maintain apH of at least 9.3 in the solutionin` order to decompose, i

the ferro-ferri color bodies.`

Brieily, "the present invention in` a preferred embodiment contemplatesmaintaining the hydrogen ionconcentration of a crude sodium ferritcyanide solution at a pH of Aat 1east`9.3 or higher,

without regard to an upper limit of pH Which `may range throughglO andeven higher, a pH of 11.2 having been found vto resultin little or nodetrimental effect. This alkaline solution isthen heated to notexceeding 65o` c., after which it is y cooled to approximately 15 C. andferrie hydroxide andsuch insolubles as sodium` ferro-` cyanide areremoved by filtration. The filtrate is a; claried solution `of sodiumferricyanide l strippedof the blue-greenfcomplexes and can be `used assuch or as an intermediate for the pro# duction of such valuablecompounds as potassium A ferricyanide, or adouble alkali metalerricyanide `bytreacting the nltrate with such salts as potas- "siumchloride and `double calcium-potassium `or` calcium-ammoniumferricyanides .byv adding calcium' chloride together with potassiumchloride ,55`

or ammonium chloride, respectively.`

Their re-` Example 1 A crude sodium ferricyanide solution containing 27%of that salt and 4% of sodium chloride and having a pI-I of about 8.3was treated with aV 25% caustic soda solution with agitation until theresulting solution had a pI-I of 9.5. 'Then 0.75%' excess of causticsoda based on the weight of the batch being treated was added to insurethe de-r composition of the ferro-ferri complexes. This reaction mixturewas heated to not exceeding 65 C. The color of the initial crudeferricyanide solution was an opalescent green, due to the co'mplexespresent, whereas after the caustic soda treatment and a iiltering stepto remove ferrie hydroxide, sodium ferrocyanide and other insolubles,the color was a clear ruby red. The removal of these undesirableproducts and the subsequent filtration step are greatly facilitated bycooling the reaction mixture to 15 C. prior to ltration. The filter cakemay be discarded or treated for subsequent by-product recovery.

Example 2 100 cc. of an aqueous solution of crude sodium ferricyanidecontaining 27 to 28% of the solute and yhaving a dark 'green cast and apH of 8.4 to 8.7 vwas mixed with a suiicient quantity of a 25% aqueoussolution of sodium hydroxide to to raise the pH of the mixture tobetween 9.3 y

sodium ferricyanide was stable and underwent no further color changeeven on prolonged Stor-- age. The clear ltrate, free of colloids,contained sodium ferricyanide in such condition as to serve asa valuableintermediate even though some sodium chloride and sodium hydroxide werepresent. It could be used forthe preparation of 'simple ferricyanidesuch as K3Fe CN 6 as well as mixed ferricyonide salts such as thepotassium and sodium double salts which are now nding such favorable usein the blue printing arts be'- cause of their content of 105% offerricyanide compared to potassium ferricyanide as a basis.

. Example 3 A crude solution of ferric'yanide containing from 27 to 28%of sodium ferricyanide was ad- .justed so as to have a pH of 11.85 byadding sos dium hydroxide. These reactants were then heated at 65C. forseveral minutes during which interval the color changed from blue-greento bright blood-red or ruby red. A red-brown precipitate of ferrichydroxide is formed. Cooling the solution to about 15 C. facilitates theprecipitation and removal of any ferrocyanide presltration in 'acentrifuge.

. ruby red solution, 'was clear and suitable for fur-1 ther reaction, aswith potassium chloride in order to obtain commercially useful products.

Example 4 A series of runs were made to determine the optimum`concentration of sodium hydroxide necessary for eecting a commerciallyfeasible destruction of the complex ferro-'ferri com- Y Y Grams of I Percent Grainsof 27-?8% Time for color N aOH 10% NaOH N agFe(GN) changesolution 1 0. 52 5. 5 1'00. 0 No change after y; hour.

2 0. 75 7. 5 02. 5 6 minutes.

3 1. 00 Y10.0 90. 0 3% minutes.

'4 1.50 15. 0 S5. 0 1 minute.

5 2. 00 V20. 0 80. 0 Do.

6 2. 50 '25. `O 75.0 Do.

The solution to which 0.52% of NaOH was added showed no change in coloreven after 1/2 hour heating at 65 After an additional 1/2 hour heatingat the only change observed was the formation o'f white curdswhile thesolution assumed a brown cast. u Apparently additions ofY up to 0.52%NaOI-I are insuicient to destroy the iron complexes, whereas additionsof 1 to 1.5% of sodium hydroxide are sucient, although more may be addedwithout unduly impairing the enicency of the process, as shown in thefollowing eX'a'lrl'llly Example 5 The high efiiciency of the caustictreatment is well illustrated by the following example showing therelatively small amount of ferricyanide destroyed. or lost during thecaustic treatment.

Percent of original i'erricyanide remaining Sample taken Before causticaddition.;

'any decomposition of the `ferricyanide"by the alkali 'i-'s effectedvirtually in therstfew minutes. .After a `considerable time `interval:the `amount of ferricyanide lost vbecause of this puriiication stepamounted to but 51.5% of the total 'presentfin'soluticvn` ,leaving asthe lter Y Caustic treated K3Fe(CN)g solution...

n Eample'xgi 'i ..1900 parts of az'z` to 23% aqueous soiution fofNa3Fe(CN)s andl100 to 200 parts of wash water previously used `on adouble alkalimetal ferri-` cyanide `iilter cake `together with ,a`suflicient `amount of a caustic soda solution to give a 1% NaOHsolution, were heatedto.,65 C, with the aidof a steam jacket. Theheating was `con7 tinued with 'agitation until theliquor became a ibrightred` color, adding more caustic if necessary.

Thisl reaction product was cooled and the diges-` tionof the ferricomplexes `was haltedby apply` ing cold water to the jacket-and using avacuum forevaporating andcooling the` solution to"20 to 25 C. The'resultantproduct was a slurry in`- cluding among other,` things, theprecipitate `forniedirlom thedecomposed color bodies. This wasreadilyltered and separated in a `centrifuge cake,` sodium ferrocyanideand small amountsof ferrichydroxide.

The ltrate, containing principally Na3Fe(,CN) s,

fil

is a highly desirable intermediate ,for thepreparation of 'simple andmixed `ferricyanides as ferrocyanide `remained, soluble in `the filtrateand hence could be] separated completelyby `subsequently recrystallizing'the ferricyanide. gaveya verypure ferricyanideproduct. `Simultaneously,Ithe removal ofthe blue-green color bodies and the recovery of .astableferricyanide compound was effected. f c

, Y.In4 addition tol the` crude `ferricyanides tioned more specicallyinthe aboveexlamples various other alkaline. forming metal`ferricyanidesas `Well as` ammonium ferricyanides can be puriiiedwithlthe aid of` an alkaline rsubstance such asan alkaline earth hydroxide,an alkaline salt, an alkali 'metal hydroxide, andthe like. Among suchcompounds are the variousmixed alkaline forming ,metal andammoniumferricyanides, or thewdouble salts such ascalcium `potassiumferricyanide calcium sodium ferricyanide, calcium ammonium ferricyanideand the like, each maybe reacted with a corresponding alkaline substanceto remove undesirable impurities and to give a ltrateand hence a producthaving such desirable properties as thoseabove described; It: is to beparticularlynoted that althoughfor easeborf descriptionfand, economy of`production sodium hydroxide Waschosen as the most fre-1 duentlyusedalkalinesubstance for illustrating specific embodiments of the processof` this `in` Fe(OH)3, and `other precipitated materials. The

iiltrate was a .olearbright ruby red ferricyanide `solution suitable forfurther,reaction;` f

. Examples 570 grams of aA 15% aqueoussolution of crude ammoniumferricyanide, `(NHi)ale(`CN)s, was heated to 65 C. and 30 "grams of a28% solution ofiNHa was added. These reactants were heated at 65 C. forseveral minutes during which interval the color changed `from blue-greento bright blood-red or rubyred.` Flocs of red-brown iFe(OI-l)isettled`out. The suspension was cooled Otherprecipitatedmaterials. The ltratewlasa clear",` bright ruby-red solution offerricyanide suitable for further reaction. Example 9 In orderto showmore clearlythe advantage 'ot the process of this invention from oneparticular point of view namely purity of product,

two runs were made wherein mixed ferricyanides` were prepared fromsodium ferricyanide and po-` tassium chloride. The product had thefollowing t content of ferriand ferrocyanides;y

"Calculated as equivalent percent of KzNaFe (CN)s i FerricyanideFerrocyanide t EzampleA Untreated KQFe(CN)5 solution...`. Filtrate fromcrude KiNaFe(CN Crystals of crude K2NaFe(CN,-e

Example (b) .cap

eee

i purifying ferricyanides in aqueous solution which vention variousothersmay be used. Thus,potas slum` hydroxide, potassium` carbonate,sodium carbonate as well as .the` more` complex ammo-` nium compoundscan be used to purify suchicom-` pounds as `the `sodium and/or`potassium ferri-V cyanides and, various` other alkaline `forming metaland ammonium ferricyanides in addition to those given above.; i

It is to be understood that the examples given are merely illustrativeand notlimitative of the invention which isito be `construed broadly andI restricted` solely byfthewscope ofthe appended claims. H u iWe-claim.: i 1. `The `method of removing color bodies from and purifyingaferricyanicle in aqueous solution which comprises reacting an alkalinesubstance therewith at a pH of at least`9.0` and at a temperature "of`substantially C., removing the color bodies as `ferrie hydroxide andrecovering the claried ferricyanide solution.`

2. The method of claim l inwhich the alkaline substance is an alkalimetal hydroxide. 3. The method of claim 1 in` which'the substanceissodium-hydroxide. l

l 4. The method of claim 1 in which the alkaline substance isammoniumhydroxide.

5. `A method of removing color bodies from and comprises adding analkaline substance untilthe pH is at least 9.0, reacting the mixture ata tem,- perature of substantially 65 C., removing the i color bodies asprecipitated ferrichydroxide, and recovering `the clarified solution.

6. A method of removing color bodies from and` purifying ferricyanidesin aqueous solution which comprises adding an alkaline substance untilthe pH is at least 9.0, reacting the mixture at atemi perature ofsubstantially 65 C., coolingthereaction `mixture,removing color bodiesas ferrocyanides and ferric hydroxide,*and recovering the `clarifiedsolution. t l,

`7. The method of removing color bodies from ,andpurifying lsodiumferricyanide in aqueous solution which "comprises reacting sodium`hydroxidetherewith at a pI-I of at least 9.0 `and at Thus, asiaresult'of [the caustic treatment, the

alkaline a temperature of" substantially 65'" C., removing the colorbodies as a precipitate offerric hydroxide and other insolublesformedand, recovering the clarified sodium ferricyanid'e solution,

8. A method of removing colorbodies .from and purifying ferricyanides inaqueous solution which comprises adding ammonium hydroxide. until the pHis at least 9.0, reactingthe mixture. at` a temperatureofsubstantially65' C., removing the color bodiesl as ferrie;hydroxide,-and,extracting the clarified ferricyanide solution- 9i, Themethod of removing. color bodies from and purifying a double salt of a.ferricyanide in aqueous solution which comprises adding, an. alkalinesubstance until the pH is at leasty 9.0, re:- acting the mixture at atemperature ofsubstan.- tially 65 C., cooling the reaction: mixture,re.- moving color bodies. as ferrocyanidesand 'ferrie hydroxide, andlextracting the; claried ferricyanide double salt.`

10; The method* of removing color` bodies from and' purifying a doublealkali metal salt of a. ferricyanide in aqueous solution whichcomprisesiadcl.- ing an alkaline substance until the pH is; at` least9.0, reacting the mixture at. a tempera-ture of stantially 65 C.,cooling the reaction mixture,

removing color bodies as ferrocyanides-and ferrie hydroxide, andextracting the claried. ferricyanide double salt.

12. The methodv of removing color-bodies from and purifying a doublesaltL of a crude alkaline forming metal and ammonium ferricyanide whichicomprises adding an alkaline substance until the pH is at least 9.0,reacting the mixture atatemperature of substantially 65' C., cooling`the reaction mixture, removingfcolorfbotdi'es as ferroGy-- mixture at atemperature:ofsubstantially 65 C., cooling the reaction` mixture,removing color bodies as ferrocyanides and ferrie hydroxide, and ex.-

tracting. the claried ferricyanide double` salt.

14. The method of removing color bodies from and purifying ferricyanidesin aqueous-solution which comprises adding an alkaline, substanceuntil'v the pH. is at least 9.0, reacting the mixture, at a temperatureof substantially 65 C., cooling the reaction mixture to about C.,removingz color bodies as ferrocyanides and` ferrie hydroxide, and

` extracting the clariiied f erricyanide solution;`

anides and: ferri'c. hydroxide, and-` extracting; the

stance until the pH; is. atleast 910;, reacting the 15; The method ofremoving color bodiesfrom and purifying yferricyanides in aqueoussolution which.. comprises adding an, alkaline substance until the pI-Iis at least 9.0', reacting the mixture at a temperatureof substantiallyC., cooling the reaction mixture and halting the digestion y of theferro-ferri complexes by vacuum evapora; tion, removingcolor bodies asferrocyanides and ferrie hydroxide, andextracting the clarified fer#ricyanide solution. f

16. The method. of removing color bodies from and purifyingferricyanides in aqueousA solution which comprises the steps of treating'a crude alkali metal ferricyanide solution containing at least. 25%thereof. with a caustic. soda. solution until the pH is at least 9.0 andheating the mix.- ture to` a temperature `of substantially 65 C, toinsure the decomposition of the ferroferri complex color bodies, coolingthe resultant mass-to substantially 15 C. to facilitatethe precipitationof thecomplex color bodies without freeing the alkali metal ferrcyanidefrom. solution separating the color bodies as. a. ferrie hydroxide andrecovering the clarified alkali metal ferricyaniide solution.

17. The method of removing color bodies` from and 'purifyingferricyanides in aqueous solution which comprises the steps of treatinga crude sodium ferricyanide solution containing at least 25% thereofWith a caustic soda solution until the pH is substantially 9.5.- andheating the mixture toa temperature of substantially 65 C. to insure thedecomposition of the ferroferli complex color. i

